The present invention relates to optical exposure systems and optical processes for aligning optical alignment layers and liquid crystals with light.
Liquid crystal compounds are used in human and machine readable displays, finding applications in instrument controls, such as those in motor vehicles, avionics, medical devices, watches, portable computers, and desk top computer monitors. Common to each of these products is a liquid crystal layer disposed between a pair of substrates coated with a polymeric alignment layer. The polymeric alignment layer controls the direction of alignment of the liquid crystal medium in the absence of an electric field. Usually the direction of alignment of the liquid crystal medium is established in a mechanical buffing process wherein the polymer layer is buffed with a cloth or other fibrous material. The liquid crystal medium contacting the buffed surface typically aligns parallel to the mechanical buffing direction. Alternatively, an alignment layer comprising anisotropically absorbing molecules can be exposed to polarized light to align a liquid crystal medium as disclosed by Gibbons et al, in U.S. Pat. Nos. 5,032,009 and 4,974,941 "Process of Aligning and Realigning Liquid Crystal Media".
Most liquid crystal devices, including displays, have a finite pre-tilt angle, controlled, for instance, by the mechanical buffing of selected polymeric alignment layers. The liquid crystal molecules in contact with such a layer aligns parallel to the buffing direction, but is not exactly parallel to the substrate. The liquid crystal molecules are slightly tilted from the substrate, for instance by about 2-15 degrees. For optimum performance in most display applications a finite and uniform pre-tilt angle of the liquid crystal is desirable.
Recently a process for optical alignment with pre-tilt using polarized light has been described by Gibbons et al in U.S. patent application Ser. No. 08/624,942 (filed Mar. 29, 1996). In that process, a material absorbs a source of radiation that is polarized. Several examples of various optical alignment layers are described in Gibbons et al. The layers are subsequently aligned at a specified angle with respect to the polarization and mediates the alignment of a liquid crystal in contact with the alignment layers. Pre-tilt of the liquid crystal can be achieved by controlling the angle of incidence of the ultraviolet radiation. This is most often achieved through multiple exposure steps.
Another process for using polarized light to expose an alignment material to orient liquid crystals with pre-tilt is described by Kobayashi et.al., in European Patent Application EP 0742471. Kobayashi, et.al., consider exposures of a substrate to linearly polarized light in two steps, the second step with polarized light containing components perpendicular to the first exposure step. The process also claims the use of unpolarized light as part of the process. Finally, Kobayashi, et.al., discuss the possibility of using elliptically polarized light as a more general description of linearly polarized light. In all discussions of said process, the angle of incidence of the light does not consider the divergence of the light and consequently treats only the case of collimated light. As such, any optical system used for exposure in said process is required to produce highly collimated and highly polarized light over the entire area of exposure.
There is a need for an optical exposure system that can deliver polarized light over large areas. There also is a need for an optical exposure system and processes for inducing pre-tilt in a liquid crystal medium in a single exposure step. To realize these objectives the inventors have found that the degree of polarization and collimation are parameters that can be adjusted in an optical exposure system and which play an important role in exposure processes. Until now, optical exposure systems for alignment materials have been designed to achieve both highly polarized and highly collimated radiation. It is now possible to control these requirements to appropriate levels and achieve improved performance and new exposure geometries.
An optical exposure system and processes that makes use of partial polarization and partial collimation of components is disclosed herein that has distinct advantages over systems that are fully polarized and fully collimated. The optical exposure system is useful as production level optical exposure system.